1. Field of the Invention
The present invention pertains to a method for growing a single crystal by crystallizing the single crystal from a float zone, the float zone being inductively heated and the crystallizing single crystal being rotated in a direction of rotation and the direction of rotation being reversed at intervals in accordance with an alternating plan which severely limits the period of the state of rest between alternating rotations.
2. Description of the Related Art
The invention pertains to the zone melting method or float zone method, or “FZ” method for short. It is often used for producing single crystals from silicon, which in turn are processed further into semiconductor wafers, from which electronic components are produced. Carried out in the classic way, a polycrystalline feed rod is melted at the lower end and a monocrystalline seed crystal is held against the melt and slowly lowered. In the further course of the method, molten material crystallizes on the seed crystal to form a single crystal. The seed crystal and the single crystal are located on a shaft, which is in connection with a drive which allows for the rotation, lifting, and lowering of the shaft. The volume of the melt is increased by further melting and lowering of the polycrystalline feed rod to form a float zone, which extends between the feed rod and the single crystal crystallizing on the seed crystal. The float zone provides the material necessary for the crystallization of the single crystal. Continued melting of the polycrystalline feed rod causes the single crystal to grow until the feed is exhausted.
It is not necessary to use a feed rod as a feed. Instead of a feed rod, fragments or granules may also form the feed. The feed does not necessarily have to be polycrystalline, either. US 2005/0188918 A1 and US 2009/0223949 A1 describe methods in which granules are used as the feed.
It is of advantage to rotate the crystallizing single crystal and to reverse the direction of rotation at intervals in accordance with an alternating plan. This measure, referred to as alternating rotation, brings about a more uniform distribution of dopants in the single crystal and promotes the growth of the single crystal in the desired cylindrical form. US 2003/0024468 A1 describes an alternating rotation that is carried out on the basis of a specific alternating plan.
The float zone is inductively heated and stabilized. For this purpose, an induction heating coil is arranged between the feed and the crystallizing single crystal. The induction heating coil is usually an almost annular flat coil with one turn, and the float zone extends through a central hole in the induction heating coil.
The region of the gap of the induction heating coil where the ends of the turn approach each other and the current supply is located, is critical, because the electromagnetic field generated by the induction heating coil is much stronger there than in the other regions of the induction heating coil. This fact can put at risk the undisturbed progress of the crystallization of the single crystal. In DE 2538831 it is proposed to allow the ends of the turn to overlap in order to reduce the field strength. According to DE 10 2011 122 381 A1, a problem in the crystallization of a conical portion of the single crystal that is caused by the lack of uniformity of the induction heating coil can be avoided by the alternating plan for the alternating rotation providing that the amount of rotation in one direction of rotation is not less than one revolution.